Check fingerprint client against top activity in auth callback

Due to a race condition with activity task stack broadcasts, it's
currently possible for fingerprint authentication to succeed for a
non-top activity. This means, for example, that a malicious overlay
could be drawn in order to mislead the user about what they are
authenticating for.

This commit addresses the issue by adding a check to the
onAuthenticated() callback of the fingerprint HAL result controller to
ensure that the authenticating activity is on top at the time of
authentication. Otherwise, the pending authentication will be canceled.

Test: Follow steps from b/159249069:
1. Install com.pro100svitlo.fingerprintauthdemo from the Play store.
2. Install the PoC attack app from b/159249069.
3. Start the PoC attack app and press the "Launch PoC attack" button.
4. Use fingerprint to authenticate while the overlay is showing.

Before: Authentication succeeds, and a new activity is launched.
After: Authentication is canceled, and no new activity is launched.

Bug: 159249069
Change-Id: I289d67e5c7055ed60f7a96725c523d07cd047b23